In response to security threats in transportation and public spaces, inspection of people for weapons and other types of contraband is becoming common at security checkpoints at, for example, airports, train stations, sporting events, concerts, federal buildings, and other public and private facilities. Systems using non-ionizing radiation, such as terahertz and millimeter wave, for imaging of concealed weapons, contraband or other items of interest are known. Such systems are described in, for example, U.S. Pat. Nos. 6,972,714, 6,965,340, 7,112,775, 7,224,314, 7,283,085, 7,333,055, 7,327,304, 7,280,068, 7,298,318, and 7,525,500 and U.S. Patent Appl. Pub. Nos. 2005-0270220, 2006-0109160, 2006-0214836, 2008-0161685, 2008-0094301, 2008-0094296.
In particular, U.S. Pat. No. 6,965,304 discloses a security inspection system having an array of antenna elements each being programmable with a phase delay to direct a beam of microwave radiation towards a target. The antenna elements may be binary phase-shifted with a non-ideal switching device, or continuous phase-shifted with a variable capacitor. U.S. Pat. No. 7,283,085 describes separate scanning and receiver arrays. U.S. Pat. No. 7,298,318 describes a system in which antenna elements are capable of being programmed with an additional transmission coefficient to receive reflected microwave radiation. US2006/0214838 describes a system in which phase shifts may be altered in order to optimize a parameter. Patterns (115) are stored in a computer readable medium (110).
In some such imaging systems there is electronic control of an array of antennas to focus radiation such as millimeter wave energy on a desired spot and to collect the resultant reflection from this focus point. The focus point can be stepped across a three-dimensional volume in order to provide an image of what is contained in the volume.
In order to step the focus point across the scan volume control information (known as the “pattern”) needs to be provided to each of the antennas in the array for each point in the volume. In order to provide good resolution and a good quality image two requirements are a large number of antennas and a small step size. These two requirements in turn create a requirement for a large amount of pattern data.
The three-dimensional step size (i.e. distance between focus points) may for example be 4 mm×4 mm×10 mm. An example scan volume of 1.1 m×2 m×1 m yields 13.75 million focus points for which pattern data must be stored. Using a coarse control mechanism where 1 bit of information is required to be stored for each antenna for each focus point yields a total pattern size of 67 GBytes for a system containing 42336 antennas (1 GByte=1024×1024×1024×8 bits).
In current systems this information is held in the system by including a large amount of distributed non-volatile memory. This non-volatile memory is “loaded” off-line outside the system and placed in the system before start-up or loaded in a setup mode at system start prior to use. The pattern data may for example be stored in Compact Flash memory from which the processor reads the data and writes it to SDRAM buffer memory, and in turn feeds the antenna array from this buffer memory.
Using non-volatile memory in the system places limitations on system operation and calibration options due to the slow access times of this type of memory. The invention is directed towards providing a simpler imaging system architecture, and/or simpler system modification, and/or improved real time response to dynamic commands during operation of the system.